This invention evolves from principles underlying the transit time oscillator (TTO) concept and the klystron wherein an electron beam interacts with an oscillating electric field to amplify or generate microwaves.
In its simplest form, a transit time oscillator (TTO) is a pillbox cavity through which an axial high energy uniform electron beam passes. The pillbox cavity has natural modes of oscillation determined by its dimensions. When the transit time of the electrons in the cavity is slightly greater than a natural period of the cavity, the beam will experience both signs of the alternating electric field during transit. Under these conditions, the electron beam can give up kinetic energy to those naturally occurring cavity modes and the beam becomes unstable. This transfer of energy from the electron beam generates within the cavity an electromagnetic field oscillating at the natural frequencies of the cavity. The growth rate of the instability can be estimated by the relative amount of the exchanged energy to the total electromagnetic energy in the cavity. Growth rates of the instability are enhanced by operating near the space charge limit of the beam. Thus, transit time oscillators in which the current is near the space charge limit should exhibit rapid growth of beam instability. Eventually the instability saturates when within the cavity, the integrated electric field along the beam equals the beam energy. Once saturation occurs electrons will be stopped and even reversed because the field opposes the motion. However, during the alternating phase the electron beam passes through the cavity and is actually pushed by the alternating electric field.
A klystron takes advantage of the phenomena wherein some of the electrons are retarded and others are accelerated by externally driven oscillating cavity fields. A klystron allows this velocity modulated electron beam to drift in free space. In the drift space, the separation between beam bunches becomes larger so that distinct electron pulses are produced. Because the length of klystron tubes are typically on the order of meters, an external magnetic field is applied to keep the electron beam on axis.
As electron beams become more relativistic, the growth rates of the instability diminish because it becomes increasingly difficult to alter the beam's velocity. To overcome the restraint posed by relativistic beams, two methods have been proposed. One is to use a non-relativistic ion beam, which can achieve much higher energies. Another proposed method to reduce the constraint is to deflect the beam transversely rather than longitudinally. This is the "Transvertron" concept and is reminiscent of the beam breakup instability observed in accelerators.
The TTO remains a concept because of several constraints which have not been practicably solved. In general, for the transit time to be longer than the modal period, the pillbox cavity must have a small radius and long length. As an example, these electron beam devices typically are used in microwave generation and amplification. For microwaves with a frequency of approximately 1 GHz or, equivalently a free space wavelength of thirty centimeters, and with a 200 keV electron beam, a TTO would require a radius of 11.5 centimeters and a length of 23 centimeters in order for the beam to experience a reversing electric field during its transit time in the cavity. The distance a high current beam can travel, however, is limited both by its tendency to pinch and by its own space charge. Thus, as in a klystron, an externally applied magnetic field would be required to keep the beam from pinching but space charge limitations will still restrict the total current.
One device which overcomes the space-charge effects of prior art microwave devices is taught in U.S. Pat. No. 4,733,133, entitled "METHOD AND APPARATUS FOR PRODUCING MICROWAVE RADIATION" to Dandl. This device illustrates the increasing complexity of microwave generation devices and methods. The invention implements an electron plasma confined by an externally applied magnetic field within a small space. The method further employs a complicated arrangement of magnetic coils to shape that plasma into annular dimensions and then adiabatically compresses that plasma to generate microwaves.
A variation of the standard virtual cathode oscillator based on a radially inward cylindrical geometry which takes advantage of the space charge limit of relativistic electrons is proposed in U.S. Pat. No. 4,751,429, entitled "HIGH POWER MICROWAVE GENERATOR" to Minich. In this instance, electrons are emitted from a hollow cylindrical velvet-lined real cathode through a coaxial anide onto an inner collector electrode. A virtual cathode is formed between the anode and a cylindrical collector electrode and this virtual cathode will experience spatial and temporal oscillations which generate microwaves. Additionally, electrons reflex back and forth between the real and the virtual cathodes which also generate microwaves. Typically, virtual cathode oscillators are low efficiency devices.
It has been noted that an electron beam can be modulated by an external radio frequency source. Taking advantage of this phenomena, J. Krall and Y. Y. Lau, "Modulation of an intense beam by an external microwave source: Theory and simulation" APPL. PHYS. LETT. 52 (6), Feb. 8, 1988, pp. 431-433, have shown how an electron beam traveling in close proximity to cavities already pumped with radio frequency energy will amplify that radio frequency power with a high degree of phase and amplitude stability.